The present invention relates to the preparation of starch based corrugating adhesives and, more particularly, to the preparation of starch based corrugating adhesives using as the aqueous medium for the adhesive a mixture of fresh water and recycled unfiltered wash water from the corrugator press and/or the flexographic printing press wash processes.
In the corrugated paperboard industry, corrugator presses are used to glue together sheets of paper with starch based adhesives to form paperboard products. Flexographic printing presses are extensively used to print various information on the paperboard products. Both these corrugator presses and these flexographic printing presses must be periodically cleaned by flushing with large quantities of water. The waste wash water from the cleaning of the corrugator presses contains unused starch adhesive, machine oils and paper lint. Similarly, the waste wash water from the cleaning of the flexographic printing presses contains contaminants such as ink solids, detergents, machine oils and paper lint. Ideally, the waste wash water from the corrugator presses and the flexographic printing presses should be recycled for use elsewhere in the plant rather than merely being discarded. A preferred use for this waste wash water would be as part of the aqueous medium used in preparing the starch adhesives for the corrugating process. Unfortunately, the presence of contaminants, particularly ink solids and ammonium ions in the flexographic printing press waste wash water, have prevented use of this waste wash water in the preparation of the starch based adhesives without prior treatment to remove these contaminants.
The conventional two-phase process for preparing corrugating adhesives, as well as various modifications thereof, is described in detail in a 1977 TAPPI monograph entitled "Preparation of Corrugating Adhesives". In the typical two-phase process described therein, the starch based adhesive is prepared in two separate portions, i.e., a cooked starch portion referred to as the "carrier starch" and an uncooked portion referred to as the "raw starch".
Typically, the carrier starch is prepared in a first tank by adding a first amount of fresh water along with a desired amount of a first starch, typically a corn starch either in raw or modified form, and the mixture agitated to form a starch slurry. Caustic soda, normally as a 50% sodium hydroxide solution, is then admixed into this starch slurry to chemically cook the starch thereby producing the carrier starch portion.
In a second tank, the raw starch portion is prepared by adding a second amount of fresh water along with a desired amount of a second starch, typically an unmodified, i.e., raw, corn starch, and agitated to form a raw starch slurry to which a borating compound, typically in the form of boric acid or borax, is also added. After the raw starch portion has been thoroughly mixed, the prepared cooked starch portion is added thereto and thoroughly mixed therewith to yield the product starch adhesive typically having a starch solids content of about 17 to about 25 percent by weight and a viscosity in the range of about 150 to about 250 centipoises.
This finished starch adhesive is passed to a relatively small, typically 100-150 gallons capacity, storage tank from where it is circulated to the corrugator on demand. Because the finished starch adhesive is quite susceptible to thickening or thinning and viscosity breakdown, the supply of finished starch adhesive is deliberately held at low levels and fresh starch adhesive is almost constantly being generated and used before it breaks down. Further, it is customary to keep the finished starch adhesive in circulation and to maintain the finished starch adhesive at a constant temperature of about 100.degree. F. to about 105.degree. F. (about 37.5.degree. C. to 40.5.degree. C.) to forestall thickening or thinning.
As noted previously, conventional practice is to use fresh water only as the aqueous medium in preparing starch adhesives even though the desirability of disposing of waste wash water by using it in the starch adhesive preparation has long been recognized. One proposal for using flexographic printing press waste wash water is presented in U.S. Pat. No. 3,970,467. As disclosed therein waste wash water from the flexographic press is treated to remove ink solids and ammonium ions from the flexographic press waste wash water and then pass the ammonium ion free water to the starch adhesive preparation tanks. It is stated in U.S. Pat. No. 3,970,467, that when untreated flexographic press waste wash water was used as a replacement for fresh water in preparing the cooked (primary or carrier) starch portion of the adhesive formulation, aggregates of coalesced starch were formed which resulted in an unworkable adhesive. It is further stated in U.S. Pat. No. 3,970,467, that even when untreated flexographic press waste wash water was used as a replacement for fresh water only in preparing the uncooked (secondary) starch portion, the resultant adhesive formulation exhibited unacceptably higher viscosities and gel points than adhesive formulations produced with fresh water or treated flexographic press waste wash water. Unfortunately, the additional process step necessary to appropriately treat the flexographic press waste wash water add further complication and cost to the adhesive preparation process.